The invention relates to a method for processing nitrogen-concentrated effluents, by oxidizing ammonium to nitrites followed by a denitration of the nitrites to nitrogen gas in a sequential biological reactor in which the reaction phases are fractionated, a method whereby an inflow volume to be processed is poured into the reactor containing nitrifying bacteria, the operating conditions being provided to privilege the action of the nitriting bacteria and to inhibit the action of the nitrating bacteria to the maximum, comprising processing cycles with at least one aeration phase to cause the nitrification, followed by a phase in which the aeration is stopped and a carbon source is introduced into the reactor for converting the nitrites to nitrogen.
The invention relates more particularly to the processing of effluents, in which the nitrogen concentration is higher than 100 mg N/l.
Processing of Nitrogen of Concentrated Effluents
Many pollutant processing installations are faced with growing difficulties associated with the control and the processing of their nitrogen-containing, chiefly ammoniacal releases, in the host environments. The ammonia is liable to cause environmental damage such as the oxygen depletion of aquatic environments, toxicity to fish, or eutrophization processes. To limit these impacts, the regulations impose increasingly stringent release limits, which often entail the costly revamping of existing processing stations.
One of the main methods for processing nitrogen is the biological method of nitrification/denitrification, whereby the ammonium is oxidized in two steps in aerated conditions, first to nitrites and then to nitrates, and is finally reduced to nitrogen gas in anoxic conditions. In the case of concentrated effluents, it is possible, by controlling various parameters, to short-circuit this biological process by carrying out a partial nitrification to nitrites, which are then directly denitrified. This method, also called “nitrate shunt” and already described in EP-A-826639 and WO 00/05176, is theoretically capable of reducing the oxygen inputs for nitrification by 25% and the biodegradable carbon inputs for denitrification by 40%, and also the associated production of heterotrophic sludge.
Wastewater treatment plants equipped with anaerobic digesters produce streams concentrated with ammonium (about 1000 mg N/l) in the sludge processing line which, when returned to the front end of the installation, may account for up to 20% of the total nitrogenous feed. Their specific processing would therefore serve in many cases to avoid the costly revamping of the main processing line to meet the release limits. Furthermore, a growing number of sites are forced to process the odors or offgases produced, generating condensates heavily laden with ammoniacal forms, which must absolutely be minimized. Finally, the leachates from landfills constitute a third type of effluent for which the ammonium ion represents one of the main sources of pollution.
All these effluents generally contain little biodegradable carbon, implying that the biological processing devices can be dimensioned almost exclusively for the processing of nitrogen. A biological processing of the “nitrate shunt” type would therefore allow significant cost reductions compared with conventional nitrification/denitrification.
Nitrate Shunt in SBR (Sequential Biological Reactors) Configuration
Fux, C., Lange K., Faessler, A., Huber, P., Grueniger, B. and Siegrist, H. (2003), in an article entitled “Nitrogen removal from digester supernatant via nitrite-SBR or SHARON?” appearing in the review Water Science and Technology, Vol. 48 No. 8, pp. 9-18 (2003) demonstrated the advantage of sequential biological reactors (SBR), comprising feed, reaction (aeration+anoxia), settling and removal phases, for the implementation of the nitrate shunt on nitrogen-concentrated effluents. In fact, this SBR configuration serves to apply larger volume feeds by biomass retention in the same reactor, unlike the SHARON method (patent EP-A-826639) for which the absence of biomass retention allows the specific leaching of the biomass responsible for the oxidation of the nitrites to nitrates. According to the SBR method of Fux et al, an inflow volume to be processed in a complete cycle is poured into the reactor in successive volume fractions, the complete processing cycle being divided into successive sub-cycles, each sub-cycle comprising a volume fraction feed phase, followed by an aeration phase to cause the nitrification, followed by an anoxia phase during which the aeration is stopped and a carbon-containing source is introduced into the reactor for converting the nitrites to nitrogen.
For releases such as anaerobic digester supernatants, gas processing condensates and landfill leachates, it is particularly difficult to optimize the nitrite production and reduction reactions, for two main reasons:                These releases are subject to extremely wide variations in flow rate and ammonium concentration, demanding the constant adjustment of the operating criteria to produce an effluent of constant quality.        Media heavily laden with salts and miscellaneous ionic compounds, such as those covered by the invention, are liable to have a negative effect on the sensitivity of the probes (contamination of redox probes by sulfides in particular) and to cause drifts that trigger a rapid transition to degraded timer mode.        
The development of a robust management system is therefore desirable to ensure the reliability of the processing of nitrogen of concentrated effluents by nitrate shunt in an SBR reactor.
The present invention proposes to remove the nitrogen from concentrated effluents by a nitration/denitration method in a sequential biological reactor (SBR) in which the sequence comprises a plurality of fractionated feed/aeration/anoxia phases, the number and duration of these phases, and also the addition of carbon-containing reagent, being adjusted by a series of real-time measurements in the effluent to be processed, in the release, and in the biological reactor.